Fig. 1. Schematic of the AOS in (a) FTF and (b) CE-FTF. The FTF is composed of Pt/GdCo/Pt/Ta. Equivalent transmission line mode of (c) FTF and (d) CE-FTF. In the equivalent circuit model, the FTF is equivalent to a load with a conductivity of σFTF, and the added Au layer induces a short circuit. Insets show the magnetic domain state, where the black contrast corresponds to the initial magnetization, and the white contrast corresponds to a magnetization reversal.

Fig. 2. (a) Conductivity of the FTF as a function of the thickness of the GdCo alloy, and the thicknesses of Pt and Ta layers are 2 and 3 nm, respectively. (b) Optical absorption of each layer and the total absorption of the FTF under different thicknesses of the GdCo alloy. (c) Phase diagram of the optical absorption of the GdCo alloy as a function of its thickness tGdCo and the thickness of the inserted dielectric layer d. (d) Absorption of each layer of the CE-FTF under different thicknesses of the inserted dielectric layer d with the thickness of tGdCo=8  nm.

Fig. 3. (a) Normalized hysteresis loops and (b) coercive field Hc for the FTFs from MOKE with different composition ratios of Gd. (c) Static MOKE images of the PFTF and the CE-FTF with d=150  nm after illumination with different laser fluences.

Fig. 4. Normalized ultrafast magnetization dynamics of (a) the CE-FTF with d=150  nm and (b) the PFTF triggered by the pump beam with different fluences. (c) The normalized ultrafast magnetization dynamics of the PFTF and the four CE-FTFs measured at t=100  ps under different laser fluences. (d) The normalized initial ultrafast magnetization dynamics of the PFTF triggered at the laser fluence of F14 and the four CE-FTFs triggered at the laser fluence of F6, respectively. Here, F1=0.35  mJ/cm2, F2=0.40  mJ/cm2, F3=0.45  mJ/cm2, F4=0.50  mJ/cm2, F5=0.60  mJ/cm2, F6=0.70  mJ/cm2, F9=0.65  mJ/cm2, F10=0.90  mJ/cm2, F11=0.95  mJ/cm2, F12=1.05  mJ/cm2, F13=1.15  mJ/cm2, and F14=1.25  mJ/cm2.

Fig. 5. Calculated (a) electron temperature and (b) phonon temperature of the PFTF and the four CE-FTFs under the same laser fluence.

Fig. 6. Equivalent circuit mode of the FTF, and the four-layer spin films can be regarded as four parallel circuits.

Fig. 7. Schematic of multiple reflections and interference model of the CE-FTF.

Fig. 8. Static MOKE imaging system. PP, pulse picker; λ/2, half-wave plate; P0, P1, P2, polarizers; D, dichroscope; BS, beam splitter; LED, light-emitting diode; CCD, charge-coupled device; OL, objective lens.

Fig. 9. Static MOKE images of the PFTF and CE-FTF with d=50, 100, 150, and 200 nm, respectively.

Fig. 10. Time-resolved MOKE (TR-MOKE) system. BS, beam splitter; C, chopper; λ/2, half-wave plate; P, polarizer; DL, delay line; M, mirror; L, lens; WP, Wollaston prism; BD, balanced detector.

Fig. 11. Normalized ultrafast magnetization dynamics of the CE-FTF with (a) d=50  nm, (b) d=100  nm, (c) d=150  nm, and (d) d=200  nm triggered by the pump beam with different fluence. Here, F1=0.35  mJ/cm2, F2=0.40  mJ/cm2, F3=0.45  mJ/cm2, F4=0.50  mJ/cm2, F5=0.60  mJ/cm2, F6=0.70  mJ/cm2, F7=0.80  mJ/cm2, and F8=0.90  mJ/cm2.